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Rockhounds and Rocks

Collectible Rocks and Minerals

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Rocks, contain clues to the geologic history of the earth and even the solar system.  They tell of the earth’s violent and fiery origin some 4.6 billion years ago.  They point to a process of “density stratification,” when earth’s raw materials – compelled by gravitational attraction – sorted themselves according to their density, forming a solid inner core, a molten outer core, a very hot but still generally solid mantle and a thin crust.  They recall the formation of an atmosphere and the oceans.  They show the effects of volcanic eruptions, drifting continents, fracturing and warping structures, changing climates, advancing and retreating glaciers, advancing and withdrawing oceans, flowing water, relentless winds, and meteorite impacts. 

Moreover, rocks chronicle – in a scrambled and fragmented way – the capricious and episodic development of life on earth over 3.5 billion years.  The fossil record documents the reign of single-cell microbes for three billion years, the emergence and florescence of multicellular life over the last 600 to 530 million years, and the ascendancy of invertebrates, marine vertebrates, reptiles, birds and mammals in the last half billion years.  Even with chapters missing, the rocks still tell us, says Stephen Jay Gould in a Scientific American article, that “The history of life tends to move in quick and quirky episodes, rather than by gradual improvement.”

In a rainbow of colors, a treasure-trove of crystals and a graveyard of frozen life forms, rocks offer us a tantalizing look at our restive planet, our biological predecessors and even our majestic solar system.  It’s enough to make a rockhound out of anyone.

How Are Rocks Formed?

Rocks in the crust of our planet form in three different ways.  First, molten material, or magma, from well below the surface, rises through fractures and fissures to fill underground chambers, cooling and solidifying to become granite, or it may erupt from volcanoes to blanket the landscape, cooling and solidifying as lava or as light frothy pumice.  Granite, lava and pumice bear the name “igneous” rocks.  Second, stone fragments and sediments produced, transported and deposited by water, ice and wind, working in concert with gravity, coalesce and solidify over millennia to become “sedimentary” rocks.  These include, for example, sandstone and limestone.  Third, igneous or sedimentary rocks, subjected to pressure and heat induced by compaction, structural faulting or folding, or magmatic intrusions beneath the earth’s surface, may change into a new, chemically altered form called “metamorphic” rocks, including, for instance, slate, marble or anthracite coal.  If any of the rocks reach high enough temperatures, they may melt, becoming molten, beginning anew the cycle of rock formation. 

Igneous, sedimentary or metamorphic rocks, which comprise aggregates of different mineral grains, may also contain mineral concentrations, which are uncombined native elements such as gold or silver or are compounds such as mica or feldspar.  The rocks’ mineral concentrations condense or crystallize from fluids that entrain elements and invade crustal fractures and voids. 

Collectible Rocks and Minerals

In the desert Southwest, good rocks and minerals – those stony aristocrats that you might, as a rockhound, like to collect and display on your mantle or make into jewelry – include, as a few examples, geodes, turquoise, quartz, fluorite, malachite, volcanic bombs, obsidian, fossils and, from beyond the earth, meteorites.


Geodes, among the most prized rocks for collectors, have a roughly spherical shape that typically spans several inches in diameter, and they have hollow interiors that come lined with crystals of several minerals, usually quartz of various colors.  The crystals may have formed, over millions of years, from minerals introduced by ground water infusing into cavities left by bubbles of gas once trapped within an igneous rock matrix.  The color variations reflect differing constituents of the water.  Geodes’ hard outer shells may have formed during the infusion of the water into the cavities.  You will find geodes in many places across the Southwest, for instance, near Ludlow in southern California or at the Rock Hound State Park and Spring Canyon Recreation Area in southwestern New Mexico. 

Turquoise, the iconic gemstone of the desert, typically occurs as veins or rock crusts in association with copper, which imparts a bluish color, or with iron, which imparts a greenish color.  Usually opaque with a waxy luster, “Turquoise is the rare and improbable product of an incalculable number of chemical and physical processes that must take place in the right combination and proper environment over a time span of hundreds of thousands – if not millions – of years,” according to the Southwest Silver Gallery Internet site.  Turquoise occurs at various sites across Nevada, Arizona and New Mexico.  Especially high quality turquoise comes from the Dameli Mine in east-central Nevada.  Some of the most famous comes from Cerrillos, between Santa Fe and Albuquerque, New Mexico, the earliest mining area in North America. 

Quartz – transparent or translucent, white, yellow, red, brown, black – ranks among the most common of the minerals.  It forms from the two most abundant crustal elements, oxygen and silicon.  Found in veins or sheeted zones, often in association with metal ores, quartz occurs in igneous, sedimentary and metamorphic formations as a six-sided crystal with pyramidal-shaped tips.  Its crystals may vary from a flyspeck up to a barrel in size.  They form in fissures, from elements introduced by hot water.  The amethyst variety usually holds the highest value as a gemstone.  In the Southwestern desert, quartz occurs, for a single example, in massive outcrops in Quartz Peak, southwest of Phoenix. Click here to see some quartz products.


Cubic translucent to clear fluorite crystals, which occur in a wide range of colors, fluoresce vividly under ultraviolet light (hence the name).  Fluorite, according to Patrick M. Colgan, Northeastern University, Rocks and Minerals Dictionary Internet site, may appear to have crystals within the crystals.   “A fluorite crystal,” he says, “could have a clear outer zone allowing a cube of purple fluorite to be seen inside...  One crystal of fluorite could potentially have four or five different color zones or bands.”  Fluorite can form at relatively shallow depths in sedimentary rocks such as limestone, where strong brines with the prerequisite constituents may invade the fissures.  It occurs, for a couple of examples, in the mountains around Wickenburg, northwest of Phoenix, or at the small peak called Bishop’s Cap, south of Las Cruces, in southern New Mexico.

By Jay Sharp - Go to Page 2

 




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